Introduction: Multiple visual streams arising from distinct populations of retinal ganglion cells become largely mixed in the cortex but remain segregated in subcortical nuclei. High-resolution functional magnetic resonance imaging and recently the application of super-resolution processing techniques have permitted the detailed study in humans of the internal structure of small subcortical structures such as the lateral geniculate nucleus (LGN) and superior colliculus (SC). Here we investigate the effects of sustained spatial attention.

Methods: In the first experiment, subjects fixated and performed a difficult motion coherence task that required attention to one of the two sides of a bow-tie stimulus composed of moving dot fields. As the stimulus slowly rotated in the visual field, the attended and unattended sides of the stimulus passed through the retinotopic receptive fields of LGN and SC neurons, and the attentional modulation was measured as the difference in the hemodymanic response between the two parts of the stimulus. In the second experiment, the bow-tie stimulus was composed of transient objects that differed in color and shape. The subjects' equally difficult task in this experiment was to detect particular feature conjunctions that appeared in only one side of the stimulus.

Results: The results show that both the LGN and SC are modulated by attention—the SC remarkably so, likely including both the superficial and deeper layers. No variation throughout the structure of the LGN or dependence on contrast sensitivity was noted that would suggest any differential modulation between the magnocellular and parvocellular layers, although the spatial resolution was not sufficient to determine this definitively. The strength of attentional modulation in both the LGN and SC was found to be independent of whether the task involved motion coherence or feature conjunctions, which is consistent with a purely spatial mechanism of attention.